The present invention relates to an electromechanical bicycle transmission.
In the art of the bicycle the structure of a transmission with sprockets in which a sprocket change mechanism shifts the chain upon command onto a selected sprocket of a plurality of sprockets of different diameters arranged concentrically and integral through an idling gear system with the rear wheel is well known.
Although generally the sprocket change mechanism is controlled by the cyclist through a control lever connected thereto through a cable there have been proposed electromechanical devices in which the sprocket change mechanism is operated substantially in simulation of the movement of the lever, through a gearmotor in turn operated on command of the cyclist by means of push buttons located on the handlebar. Such embodiments are however not popular because of the peculiar operation through which the sprocket change mechanisms shift the chain among the various sprockets and which makes unsatisfactory the simple replacement of the lever by an electric motor. Indeed, in a transmission using a sprocket change mechanism it can happen that during passage between the sprockets the sprocket change mechanism encounters temporary high resistances. For example, a necessary condition for correct gear shifting is that the pedals be operated for the entire duration of the transition between the sprockets to ensure correct engagement between the destination sprocket and the chain links. Irregularities or temporary interruption of pedalling during shifting leads to the rise of the above mentioned resistances in the movement of the transmission.
In manual shifting, the cyclist perceives through the mechanical connection of the lever the changes in resistance opposed by the chain to movement of the transmission and is thus able to control pedalling and the operation of the lever. With known electromechanical gear shifting devices such a feedback through the lever is completely lacking and hence, during shifting, the gearmotor often works for a period of time of variable length under conditions of excessive resistance and this can bring errors of positioning, wear and failure. In addition, the shift takes place with low speed and accuracy. These defects have until now limited the use of electromechanical transmissions in cycling.